1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly relates to an image forming apparatus in which a photoreceptor is scanned with plural light beams to form plural images on it and the plural images are overlapped to form and generate a single image.
2. Description of the Related Art
Apparatus for forming an electrostatic latent image by scanning with a light beam modulated correspondingly to an image to be formed and forming an image on a photoreceptor have been used for apparatus such as printers and copying machines heretofore, and concomitantly with commercialization of digital or color apparatus the image forming apparatus having the above-mentioned structure have been used widely. A color image is realized by forming images of different colors on a photoreceptor successively so that, for example, images of four colors (for example, C, M, Y, and K) different in color each other are overlapped on the single photoreceptor, however this method is disadvantageous in that it takes a time for forming a final color image.
To solve the problem, a so-called tandem type image forming apparatus in which the image forming apparatus has plural photoreceptors, the photoreceptors are scanned and exposed simultaneously with plural light beams respectively to form images different in color each other on the respective photoreceptors, and the color images of different colors are overlapped on a single intermediate transfer medium to form a color image is proposed. Because a tandem type image forming apparatus forms images of respective colors simultaneously, the time required for forming a color image is significantly shortened.
In a tandem type image forming apparatus, it is required to successively delay the writing start timing (modulation start timing) of light beams corresponding to respective colors synchronously with the motion of the intermediate transfer medium to overlap images (toner image) of respective colors formed on the respective photoreceptors without deviation along the vertical scanning direction (the direction perpendicular to the scanning direction of the light beam) on the intermediate transfer medium. The delay magnitude is determined based on the error of the incident position of the light beam on the photoreceptor, the error of a half circumference length of the photoreceptor, and the error of moving speed of the intermediate transfer medium between photoreceptors of respective colors.
However, the image positional deviation magnitude allowable for overlapping of images is generally at most 0.1 mm though it depends on the image data to be generated; on the other hand, the values of the above-mentioned parameters vary when the parts are deformed due to heat or external force. Even though the mechanical accuracy such as dimensional accuracy of the parts are merely improved, it is difficult to maintain the image overlapping accuracy continuously. If the magnitude of the image positional deviation is significantly larger, for example, the hue of a color varies in an area where the hue is to be uniform or the hue becomes partially different in the area to cause non-uniformity, and the non-uniform hue is regarded as the image quality detect.
To solve the above-mentioned problem, a method for correcting the positional deviation of an image in which the position of images of respective colors transferred onto an intermediate transfer medium is measured and the writing delay time is corrected correspondingly to the detected positional deviation magnitude has been employed heretofore.
In the case plural images are overlapped to form and generate a single image, the positional deviation of images to be overlapped occurs in the horizontal scanning direction (the scanning direction of a light beam), it is possible for the horizontal direction to correct the image position in a length unit smaller than the pixel interval along the horizontal scanning direction, and as the result, the positional deviation of the image can be corrected precisely.
The image position along the horizontal scanning direction of the image varies depending on the modulation start timing of a light beam in one scanning with the light beam, because usually the interval from the time when a beam position detection sensor such as SOS sensor detects the light beam to the time when modulation of the light beam is started is controlled by use of the data represented by the number of pulses of a clock signal, by employing the method in which the clock signal having the frequency higher than the frequency corresponding to a pixel interval is used and the data is corrected correspondingly to the positional deviation magnitude in the horizontal scanning direction, the image position is corrected in a length unit smaller than the pixel interval along the horizontal scanning direction.
However, because a pixel interval along the vertical scanning direction (scanning line interval) is the minimum unit for correction of the image position usually for the image positional deviation along the vertical scanning direction, a method in which the delay time of modulation start timing for correcting the image positional deviation along the vertical scanning direction is divided by the time equivalent to one scanning with a light beam to figure out the number of delay scanning lines and, if a fraction is generated, the fraction is subjected to rounding process to generate an integer, and the delay time of the modulation start timing is converted to an integral multiple of the time equivalent to one scanning with the light beam has been employed. Various rounding methods such as rounding up the fraction, rounding down the fraction, and counting fractions of 5 and over as a unit and disregarding the rest are available as described in, for example, Japanese Published Unexamined Patent Application No. Sho 64-38764 as the fraction rounding process of the number of delay scanning lines.
For example, in the case of rounding method of counting fractions of 5 and over as a unit and disregarding the rest, a fraction of the number of delay scanning lines exceeding xc2xd is rounded up and a fraction smaller than xc2xd is rounded down, as the result the writing positional deviation (=(the actual value of the number of delay scanning lines before rounding by means of counting fractions of 5 and over as a unit and disregarding the restxe2x88x92the integral value of the number of delay scanning lines after rounding by means of counting fractions of 5 and over as a unit and disregarding the rest)xc3x97scanning line interval) is suppressed to a value equal to or smaller than xc2xd of the scanning line interval. However, in the case that the number of light beams to be written is 3 or larger, because the numbers of delay scanning lines of respective light beams are not correlative each other (at least the fraction values of numbers of delay scanning lines of respective light beams are not correlative each other), the position of respective images formed by light beams deviates at most xc2x1(scanning line interval/2) along the vertical scanning direction.
The maximum positional deviation magnitude of the image is the same as the above-mentioned result for the case in which rounding by means of rounding up the fraction or rounding down the fraction is employed instead of the above-mentioned rounding by means of counting fractions of 5 and over as a unit and disregarding the rest, and the maximum positional deviation magnitude along the vertical scanning direction is approximately equal to the value of a scanning line interval regardless of any fraction rounding method selected from among known various fraction rounding methods. Therefore, the correction accuracy of the image positional deviation along the vertical scanning direction is insufficient.
It is possible to correct the image position along the vertical scanning direction in a length unit smaller than a scanning interval by use of a structure having plural respective deflection units corresponding to plural light beams to be deflected by separate respective deflection units, in which the phase of the light beam deflection scanning by means of each deflection unit is varied (for example, by varying the rotational phase of the rotating polygonal mirror, if a rotating polygonal mirror is used as the deflection unit), however the structure and the control are very complex disadvantageously.
On the other hand, the structure for deflecting plural light beams by means of single deflection unit requires a deflection unit (for example, a mechanism for tilting a mirror very small angle) that deflects the beams in the vertical scanning direction, and also the requirement bring about disadvantageous very complex structure and the control.
The present invention was accomplished to solve the above-mentioned problem and provides an image forming apparatus which is capable of reducing the positional deviation of images in the direction intersecting with the scanning direction of the light beam without significantly complex structure and control when plural images are overlapped.
In the case that deflected N (Nxe2x89xa72) light beams are emitted on an object to be scanned at the same timing and period, the maximum deviation (the state at the time when the positional deviation magnitude in the vertical scanning direction is maximized) of the position of the N light beams along the vertical scanning direction under the condition that the irradiation position along the vertical scanning direction which is orthogonal with the scanning direction of light beams is variable in a scanning line interval unit occurs at the timing when the respective positions of N light beams (each light beam is shown with xe2x80x9c◯xe2x80x9d) are arranged uniformly with a space of the interval of (scanning line interval/N) in a range equivalent to the scanning line interval as shown in FIG. 1.
FIG. 1 is a diagram for illustrating conceptually the positional relation between light beams, and if the position of a light beam is regarded as the position of an image, then the position of light beams shown in FIG. 1 corresponds to the state at the time when the positional deviation of plural images overlapped to form a single image is maximized.
The inventors of the present invention found that the maximum positional deviation magnitude of the position of N light beams was (Nxe2x88x921)/N times a scanning line interval (represented by xe2x80x9cdotxe2x80x9d in FIG. 1), that is, the maximum positional deviation magnitude was smaller than a scanning line interval, and based on this fact, the inventors of the present invention reached to the idea that it was possible to reduce the maximum positional deviation magnitude of images along the direction intersecting with the scanning direction of light beams to the value smaller than a scanning line interval in spite of the restriction that the variation unit of the modulation start timing of light beams was the time equivalent to one scanning with a light beam, and thus the present invention was accomplished.
The image forming apparatus according to the present invention, in which a photoreceptor is scanned with plural light beams respectively to form plural images thereon and the plural images are overlapped to generate a single image, has a detection part that detects a positional deviation magnitude of each of plural images in a direction orthogonal to a scanning direction with the light beams, a correction part that calculates a deviation of a modulation start timing of each light beam for correcting the positional deviation magnitude detected by the detection part, and determines the modulation start timing of each light beam based on the combination of the deviation of the modulation start timing of each light beam using the time required for one scanning with the light beam as a unit for changing the modulation start timing of each light beam, and a modulation control part that controls the modulation of each light beam according to the modulation start timing determined by the correction part.
The image forming apparatus functions to scan a photoreceptor with plural light beams to form plural images, and to overlap the images to generate a single image. As the result, for example, if the plural images are images different in color, the output image which has been generated by synthesizing the plural images is a multi-color image (a full color image if respective colors of the plural images are K, Y, M, and C).
In the known image forming apparatus, because the fraction rounding process of the number of delay scanning lines of individual light beams is performed with reference only to the value of the number of delay scanning lines to be processed, the maximum deviation magnitude of the image along the direction intersecting with the scanning direction is approximately equal to the value of a scanning line interval. On the other hand, in the image forming apparatus of the present invention, because the modulation start timing of each beam is determined based on the combination of the deviation of the modulation start timing of each light beam, for example, the position of light beams is arranged not uniformly as shown in FIG. 1 and the modulation start timing is determined so that the position of the light beams is distributed biasedly in the range equivalent to a scanning line interval (in detail, the fraction of the deviation of the modulation start timing is rounded) to thereby reduce the maximum positional deviation magnitude of each image along the direction intersecting with the scanning direction of the light beam ((Nxe2x88x921)/N times a scanning line interval wherein N is the number of light beams).
Therefore, the positional deviation of each image along the direction intersecting with the scanning direction of the light beam is reduced when plural images are overlapped. Furthermore, though the present invention may be applied not only to an embodiment in which plural light beams are deflected by a single deflection unit but also to an embodiment in which plural light beams are deflected by any of plural deflection units, particularly the former embodiment in which a single deflection unit is provided is preferable because the positional deviation of each image along the direction intersecting with the scanning direction of the light beam is reduced without introduction of significantly complex deflection mechanism and control for the vertical scanning direction of each light beam.
In the image forming apparatus, the correction part may obtain a quotient Q and a remainder R for each of the light beams other than a reference light beam by dividing the deviation of the modulation start timing of each of the light beams from the modulation start timing of a reference light beam by a time required for one scanning with the light beam, and may determine the modulation start timing of each of the light beams based on a combination of values of the remainder R for respective light beams.
Because the remainder R represents the positional deviation magnitude along the direction intersecting with the scanning direction of the image formed by delaying the modulation start timing of other light beams by a predetermined number of scanning line scanning with respect to the position along the vertical scanning direction of the image formed by the reference light beam, the positional relation of each light beam is judged based on the combination of the remainder R value of each light beam, and the modulation start timing can be determined so that the position of each light beam is distributed biasedly in the range equivalent to a scanning line interval.
Further, in the image forming apparatus, the correction part may obtain a quotient Q and a remainder R for each of N (Nxe2x89xa64) light beams by dividing the deviation of the modulation start timing of each of (Nxe2x88x921) light beams from the modulation start timing of a reference light beam by a time required for one scanning with the light beam, and based on conditions prescribed according to respective possible values of the number of the light beams having the remainder R exceeding (Nxe2x88x922) IN, may select rounding up or rounding down of the remainder R for each of the light beams to determine the modulation start timing of each of the light beams.
In detail, the correction part calculates a quotient and a remainder R by dividing the deviation of the modulation start timing of respective Nxe2x88x921 light beams with respect to the modulation start timing of the reference light beam out of N (Nxe2x89xa64) light beams by the time equivalent to one scanning with the light beam, and selects a fraction rounding process out of rounding up the remainder R and rounding down the remainder R for each light beam based on the selection condition for selecting a fraction rounding process determined correspondingly to the respective values which can be as the number of light beams having the remainder R exceeding (Nxe2x88x922)/N (if N=3, the value which can be as the number of light beams having the remainder R value exceeding ⅓ is xe2x80x9c0, 1, and 2xe2x80x9d; on the other hand, if N=4, the value which can be as the number of light beams having the remainder R value exceeding xc2xd is xe2x80x9c0, 1, 2, and 3xe2x80x9d), and then determines the modulation start timing of each light beam.
In the case that the number of light beams having the remainder R value exceeding (Nxe2x88x922)/N is a relatively high or relative low value, because the position of each light beam is distributed biasedly in the range equivalent to a scanning line interval, the selection condition can be determined so that the fraction rounding process may be selected fixedly from among the rounding down of the remainder R and the rounding up of the remainder R for each light beam. On the other hand, in the case that the number of light beams having the remainder R value exceeds (Nxe2x88x922)/N is a medium value, because it is likely that the degree of biased distribution of the position of each light beam in the range equivalent to a scanning line interval is low, the selection condition can be determined so that, for example, the fraction rounding process may be selected variably from among the rounding down of the remainder R and the rounding up of the remainder R depending on the remainder R value for respective light beams.
Because the correction part determines the modulation start timing of each light beam by selecting the fraction rounding process from among the rounding down of the remainder R and the rounding up of the remainder R for each light beam based on the above-mentioned selection condition, the modulation start timing of each light beam is determined in a short time.
In the image forming apparatus, the correction part may apply the determination process of the modulation start timing of each light beam based on the combination of the deviation of the modulation start timing of the light beam only to some light beams the number of which is smaller than the total number of light beams.
Therefore, the above-mentioned invention is applied when, for example, a color image is generated, the present invention is applied only to the image of a color which causes remarkable color deviation out of images of component different colors for forming a color image to determine the modulation start timing of the light beam, and the image quality of the generated image is improved the more.
In the image forming apparatus, the detection part may be capable of detecting the positional deviation magnitude of the image with a detection unit of 1/N scanning line interval (N represents the number of light beams to which the correction part applies the modulation start timing determination process).